Will someone knowledgeable please ACK me on this...
Reading in the current CWAP book (bottom of page 202), this frame has a broadcast group address in the RA field.
How does the specific STA that needs an ACK know the ACK in this particular frame is specifically for him?
As is the title of the frame, CF End + CF ACK the frame has two purposes. The receiver address is a broadcast because everyone needs to know that the CF is ending. The CF ACK is of course also there and every STA processes the CF ACK. The only STA that it matters to though, is the STA that just sent a frame. I don't know if this helps, but remember that this frame would be direct preceded by the data frame that the STA in question sent.
I guess to more directly answer the question - How does the specific STA that needs an ACK know the ACK in this particular frame is specifically for him?
He doesn't actually know that it is for him. What he does know is that he is expecting an ACK within a certain period of time (SIFS) and he received an ACK within that period of time. Off the top of my head, this is the only ACK type frame that has a broadcast as a receiver address. Sorry for the long reply, but I hope it helps.